The relationship between the visually perceived densities of objects in an image scene reproduction compared to those in the original scene is a critical aspect of achieving viewer satisfaction with the reproduction. Visually perceived densities of objects in an original scene or in an image reproduction of the scene include effects of viewing flare and, as is known in the art, can typically be measured using a low-flare radiometer. The theoretically correct reproduction for this purpose is generally considered to be a one-to-one relationship between the densities of the original scene and the densities of the reproduction. This is described in the book "The Reproduction of Colour" by Dr. R. W. G. Hunt, (Fountain Press, England--Fourth Ed.), specifically in Chapter 6.
In reality however, actual tone reproduction produced by practical image reproduction systems deviates significantly from the theoretical one-to-one relationship that one might logically expect. For example, in conventional silver-halide based photographic systems, the silver-halide materials inherently tend to produce a well known non-linear, "S"-shaped relationship between the viewed print density (in the case of a print system) vs. scene exposure, such as shown on page 54 of the aforementioned Hunt reference. Similar curves are illustrated in FIGS. 1 and 2 in the accompanying drawings wherein FIG. 1 shows the D-logE curve D and its corresponding instantaneous gamma curve G representative of a typical amateur film/paper system and FIG. 2 shows the same class of curves D' and G' representative of a typical professional film/paper system. The term "instantaneous gamma" as used herein refers to the instantaneous rate of change of the image density, as perceived by the viewer, versus scene exposure density. Thus the curves G and G' correspond to the incremental slope values of the curves D and D', respectively.
It can be readily seen from FIGS. 1 and 2 that the instantaneous gamma curves of the reproduced image in both systems exhibit somewhat bell-shaped curvature in the mid-tone range with significant difference between the maximum and minimum values of the curves within the mid-tone range. The major result of this behavior in these reproduction systems is that the tone mapping of the reproduced image contains somewhat of an unnatural amplification of the mid-tone information in the image with a compression of the shadow and highlight information. The visual effect is one of a degree of harshness in the image that is recognized by the viewer as not being a natural reproduction of the original scene. It should be noted here that image reproduction of a scene that is perceived by a viewer to be a natural reproduction of the scene entails consideration of the effect that any stage in the reproduction process has on the entire process from image capture to final reproduction of the image. In U.S. Pat. No. 4,792,518--Kuwashima et al, there is disclosed a silver halide color reversal reflection print material, the purpose of which is to faithfully reproduce an original image which might exist in the form of either a transparency image or a reflection print image. This disclosure describes a method for faithfully, i.e. exactly, copying a transmission or reflection original and stresses the need for linearity in the gamma curve of the reproduction material across an extended range of original image density to achieve a faithful reproduction of the original image source (slide or print). To this extent, it is similar to the basic concept of a one-to-one relationship mentioned above in connection with the Hunt publication. Such a concept however, does not recognize or suggest what is needed to provide a natural color image reproduction of an original scene taking into account characteristics of the image capture medium and viewing flare when viewing the reproduced image. The print materials disclosed in this patent would simply reproduce whatever gamma characteristics exist in the image origination materials, which at present are distinctly non-linear, without regard to whether such materials would result in a natural reproduction of the original scene.
The challenge in image scene reproduction, therefore, is to reproduce the original scene in such a manner that upon viewing the reproduction, the viewer has the impression of looking at the original scene; that is to say, viewing the reproduction should inspire in the viewer the same response as if viewing the original scene. In this sense then, the reproduction will appear natural to the viewer even though it may not be exactly faithful to the original scene, i.e. may not be an exact measurable one-to-one density mapping relationship between the original scene and the reproduction. The result would then be a pleasing reproduction that would be preferred over reproductions that do not convey that impression.
It is desirable, therefore, to provide an image reproduction system and method that offers an overall tone mapping in the reproduced image as seen by the viewer that is perceived to be a natural and pleasing reproduction of the original scene.